1. Field of the Invention
The present invention relates to a polyimide optical waveguide, and more particularly to a polyimide optical waveguide in which the polyimide, the refractive index of which is controlled by an electron beam irradiation to a desired value, is used as a core, and also to a facile fabrication of the polyimide optical waveguide.
2. Description of the Related Art
As an optical communication system is put to practical, thanks to the development of a low loss optical fiber, it is desired that various kinds of devices for the optical communication be developed. Further, there are demands for an optical wiring technique, more particularly an optical waveguide technique by which the optical devices are packaged in high density.
Generally, some conditions are required of an optical waveguide, for example, low optical losses, facile fabrication, controllable core-cladding refractive index ratio, and a high heat resistance.
As material of the optical waveguide having low optical losses, a silica-based material can be cited. As already proved in optical fiber, silica which has a good optical transmittance achieves an optical loss of 0.1 dB/cm or less at wavelength of 1.3 .mu.m, if used in an optical waveguide. Manufacture of quartz glass optical waveguides, however, presents a number of problems including a manufacturing processes lengthy in terms of time, the high temperatures needed during fabrication, and the difficulty of making optical waveguides with a large area.
To solve these problems, attempts have been made to produce optical waveguides using plastics such as polymethylmethacrylate (PMMA), which can be manufactured at low temperatures and low cost. Conventional plastic optical waveguides, however, have low resistance to high temperature. Thus, there is a demand for plastic optical waveguide having an excellent heat resistance.
Among the various organic polymers currently available, polyimides provide very high resistance to heat. Hence, these materials have been widely employed in the field of electronics, to form an insulating film between layers in the multilayer wiring or to form a flexible print board. However, there has been no example in which polyimide is applied for an optical device such as an optical waveguide.
In view of this, the inventors of the present invention have studied and developed a polyimide optical material which is applicable for an optical waveguide. When polyimide is used in optical communication applications as the optical material, there are two important points. First, its transparency in the visible and near infrared regions is excellent. Second, its refractive index can be controlled freely. The inventors disclose a fluorinated polyimide with excellent transparency in the visible and near infrared regions, in Jpn. Pat. Appln. KOKAI Publication No. 3-72528. Further, in Jpn. Pat. Appln. KOKAI Publication No. 4-8734, it is disclosed that the core-cladding refractive index ratio, as is needed for, for example, forming an optical waveguide, is quite controllable by copolymerizing such fluorinated polyimide. Furthermore, optical waveguides using fluorinated polyimide are disclosed in Jpn. Pat. Appln. KOKAI Publications Nos. 4-9807, 4-235505 and 4-235506. The control of the refractive index difference between the core layer for passing the light and the cladding layer for shutting the light is achieved by adjusting the fluorine content in the polyimide. Namely, two kinds of fluorinated polyimide having the different refractive indices for the core layer and for the cladding layer are used respectively. Therefore, there may be some problems with a kind of an optical waveguide; that is, the core layer and the cladding layer may be different in thermal properties or the birefringence.
In the conventional method of manufacturing a polyimide optical waveguide, a reactive ion etching (RIE) method, which is used in a semiconductor manufacturing process, is generally employed. The RIE method is disadvantageous in that it comprises many steps. Therefore, a facile fabrication of polyimide optical waveguides, which comprises a smaller number of steps and by which the above-mentioned method may be replaced, has been desired.
The inventors of the present invention have studied to solve the above-mentioned problems and have found it possible to change the refractive index of polyimide films by irradiating the films with an electron beam, as they disclose in Japanese Patent Application No. 4-226549.